1. Field of the Invention
The invention relates to an electrical storage systems comprising a sheet-type discrete element, to a discrete sheet-type element, a method for the production thereof, and its use.
2. Description of Related Art
Electrical storage systems have long been state of the art and include in particular batteries, but also high energy density capacitors, known as supercaps. Since high energy densities can also be realized with lithium-ion batteries, the latter are in particular also being discussed in the field of novel applications, such as electromobility, although having already been used for a number of years in portable devices such as smartphones or laptop computers. These conventional rechargeable lithium-ion batteries are in particular characterized by the use of organic solvent-based liquid electrolytes. As the latter are inflammable, they lead to safety concerns regarding the unrestricted use thereof. One way of avoiding organic electrolytes is to use solid electrolytes. However, such a solid electrolyte has a conductivity that is usually clearly smaller, i.e. by several orders of magnitude, than that of a corresponding liquid electrolyte. In order to nevertheless achieve acceptable conductivities and to be able to exploit the advantages of a rechargeable lithium-ion battery, such solid-state batteries are nowadays especially produced in the form of so-called thin film batteries (TFB) or thin film storage elements which find their use in particular in mobile applications, for example in so-called smart cards, in medical technology and sensor technology as well as in smartphones and other portable applications which require smart, miniaturized and possibly even flexible power sources.
An exemplary lithium-based thin film storage element has been described in US 2008/0001577 and basically consists of a substrate on which the electronically conductive collectors for the two electrodes are deposited in a first coating step. In the further manufacturing process, the cathode material is first deposited on the cathode collector, usually lithium cobalt oxide, LCO. In the next step, a solid electrolyte is deposited, which is usually an amorphous material including the substances lithium, oxygen, nitrogen, and phosphorus, and which is referred to as LiPON. In the next step, an anode material is deposited so as to be in contact with the substrate, the anode collector, and the solid electrolyte. In particular metallic lithium is used as the anode material. When the two collectors are connected in electrically conductive manner, lithium ions will migrate through the solid-state ion conductor from the anode to the cathode in the charged state, resulting in a current flow from the cathode to the anode through the electrically conductive connection of the two collectors. Vice versa, in the non-charged state migration of the ions from the cathode to the anode can be enforced by applying an external voltage, whereby the battery is charged.
A further thin film storage element is described in US 2001/0032666 A1 by way of example, and also comprises a substrate onto which different functional layers are deposited.
The layers deposited for such a thin film storage element usually have a layer thickness of about 20 μm or less, typically less than 10 μm or even less than 5 μm; a total thickness of the layer structure can be assumed to be 100 μm or less.
In the context of the present application, thin film storage elements refer to rechargeable lithium-based thin film storage elements and supercaps, by way of example; however the invention is not limited to these systems but may as well be used in other thin film storage elements, e.g. rechargeable and/or printed thin film cells.
A thin film storage element is generally manufactured using complex coating processes also including patterned deposition of the individual materials. Very complex patterning of the exact thin film storage elements is possible, as can be seen from U.S. Pat. No. 7,494,742 B2, for example. In case of lithium-based thin film storage elements, particular difficulties are moreover encountered due to the use of metallic lithium as an anode material because of the high reactivity thereof. For example, metallic lithium has to be handled under preferably water-free conditions since otherwise it would react to form lithium hydroxide and the functionality as an anode would no longer be ensured. Accordingly, a lithium-based thin film storage element must also be protected against moisture by an encapsulation.
U.S. Pat. No. 7,494,742 B2 describes such an encapsulation for the protection of non-stable constituents of a thin film storage element, such as, e.g., lithium or certain lithium compounds. The encapsulation function is here provided by a coating or a system of different coatings which may fulfill further functions as part of the overall design of the battery.
In addition, as described in document US 2010/0104942 A1 by way of example, under the manufacturing conditions of a lithium-based thin film storage element, in particular during so-called annealing or heat treatment steps which are necessary for the formation of crystal structures suitable for lithium intercalation, undesirable side reactions of the mobile lithium ions with the substrate may occur, since lithium exhibits high mobility and can easily diffuse into common substrate materials.
A further issue with thin film storage elements relates to the substrate materials employed. Prior art describes a multiplicity of different substrate materials, such as, for example, silicon, mica, various metals, and ceramic materials. The use of glass is also often mentioned, but essentially without further details on the particular composition or precise properties of the glass.
US 2001/0032666 A1 describes a capacitor-type energy storage which may for instance be a lithium-ion battery. Here, semiconductors are mentioned as substrate materials, inter alia.
U.S. Pat. No. 6,906,436 B2 describes a solid state battery in which metal foils, semiconductor materials or plastic films can be used as substrate materials, for example.
U.S. Pat. No. 6,906,436 B2 describes a variety of possibilities for optional substrate materials, for example metals or metal coatings, semiconducting materials or insulators such as sapphire, ceramics, or plastics. Different geometries of the substrate are possible.
In U.S. Pat. No. 7,494,742 B2, metals, semiconductors, silicates, and glass, as well as inorganic or organic polymers are described as substrate materials, inter alia.
U.S. Pat. No. 7,211,351 B2 mentions metals, semiconductors, or insulating materials and combinations thereof as substrates.
US 2008/0001577 A1 mentions semiconductors, metals, or plastic films as substrates.
EP 2434567 A2 mentions, as substrates, electrically conductive materials such as metals, insulating materials such as ceramics or plastics, and semiconducting materials such as, e.g., silicon, and combinations of semiconductors and conductors or more complex structures for adapting the coefficient of thermal expansion. These or similar materials are also mentioned in documents US 2008/0032236 A1, U.S. Pat. No. 8,228,023 B2, and US 2010/0104942 A1.
By contrast, US 2010/0104942 A1 describes, as substrate materials that are relevant in practice, only substrates made of metals or metal alloys having a high melting point, and dielectric materials such as high quartz, silicon wafers, aluminum oxide, and the like. This is due to the fact that for producing a cathode from the usually employed lithium cobalt oxide (LCO), a temperature treatment at temperatures of more than 400° C., even more than 500° C. and above is necessary in order to obtain a crystal structure that is particularly favorable for storing Li+ ions in this material, so that materials such as polymers or inorganic materials with low softening points cannot be used. However, metals or metal alloys as well as dielectric materials have several shortcomings. For example, dielectric materials are usually brittle and cannot be used in cost-efficient roll-to-roll processes, while metals or metal alloys, on the other hand, tend to oxidize during a high-temperature treatment of the cathode material. In order to circumvent these difficulties, US 2010/0104942 A1 proposes a substrate made of different metals or silicon, wherein the redox potentials of the combined materials are adapted to each other so that controlled oxide formation occurs.
Furthermore, many publications discuss how to circumvent the high temperature resistance of the substrate which is required, for example, in the aforementioned US 2010/0104942 A1. By adapting process conditions, for example, substrates with a temperature resistance of 450° C. or below can be used. However, prerequisites for this are deposition processes in which the substrate is heated and/or the sputtering gas mixture of O2 and Ar is optimized and/or a bias voltage is applied and/or a second sputtering plasma is applied in the vicinity of the substrate. This is discussed, for example, in US 2014/0030449 A1, in Tintignac et al., Journal of Power Sources 245 (2014), 76-82, or else in Ensling, D., Photoelectron spectroscopy examination of the electronic structure of thin lithium cobalt oxide layers, dissertation, Technical University of Darmstadt, 2006. In general, however, such process engineering adaptations are expensive and, depending on the processing, are hardly implementable in a cost-effective manner, especially if in-line coating of wafers is envisaged.
US 2012/0040211 A1 discloses a glass film as a substrate, which has a thickness of at most 300 μm and a surface roughness of not more than 100 Å. This low surface roughness is required because the layers of a thin film storage element generally have very low layer thicknesses. Even small unevenness of the surface may have a critical adverse effect on the functional layers of the thin film storage element and may thus result in failure of the battery as a whole. The same applies to document WO 2014/062676 A1 which claims thin film batteries using borosilicate glass or soda-lime glass. Again, information on thickness variations of the substrate are not given there.
Therefore, problems of conventional thin film storage elements are related to the corrosion susceptibility of the employed materials, in particular if metallic lithium is used, which implies complex layer structures and hence usually causes high costs, and are also related to the type of the substrate which should in particular be non-conductive but flexible, should exhibit high temperature resistance and should preferably be inert to the functional layers of the storage element used, and moreover should allow for deposition of layers preferably free of defects and with good layer adhesion on the substrate. However, it has been found that even with substrates having a particularly low surface roughness such as the glass film proposed in US 2012/0040211 A1, for example, failure of layers may occur as a result of cracks and/or detachment of the layers, as described in US 2014/0030449 A1, for example. The method for avoiding high annealing temperatures proposed therein, namely by applying a bias voltage when creating the lithium cobalt oxide layer, however, is difficult to implement in the common in-line processes for producing thin film storage elements, as already described above, so that from a process engineering point of view it is more favorable to use a substrate having a correspondingly high temperature resistance.
Another problem that is inherent to all substrate materials regardless of their exact composition relates to one of the possible handling procedures for ultra-thin glass. This so-called carrier solution consists of temporarily fixing ultra-thin glass on a support prior to or during the coating process or the transfer process steps. This may optionally be achieved using electrostatic forces, or by using an organic, detachable adhesive compound. In particular in the latter case it has to be ensured, by suitable choice of the substrate or of the carrier which are usually made from the same material, that debonding, that means detachment of the substrate from the carrier is possible. The debonding often provokes the occurrence of torsional stresses in the substrate, which stresses may furthermore be transferred to the layers deposited on the substrate, which also leads to cracks and detachment of the layers, so that as a result the layer defects caused by thickness variations of the substrate may further aggravate.